During combustion of spent liquor from the sulphate pulp industry, for instance, two partial goals are aimed at, the first being that the organic wood substance released is combusted so that its chemical energy is converted to useful thermal and electric energy and the second being that the inorganic chemicals used shall be recovered and converted to active form. This means, among other things, that the sulphur shall be recovered in sulphide form, which requires under-stoichiometric combustion, while the energy recovery requires over-stoichiometric conditions. This means that two opposing processes must be performed simultaneously in one and the same combustion chamber which, with conventional soda recovery unit technology, results in optimizing problems.
The inorganic part recovered from the soda recovery unit, the melt, is dissolved to green liquor and consists of a plurality of chemical compounds. Most of these compounds consist of sodium carbonate and also sodium sulphide, the latter constituting the first one of the recovered active digesting chemicals in the white liquor. However, the cycle is not a hundred per cent efficient and some of the digesting chemicals are obtained in inactive form, thereby constituting ballast in the white liquor.
Preparation of the white liquor is the last step in the recovery of chemicals. Here the sodium carbonate in the green liquor is converted to sodium hydroxide by the reaction with calcium oxide (CaO), i.e. caustic lime. The second one of the two active digesting chemicals in the white liquor has thus been recovered.
The process is generally termed causticizing and an insoluble sludge consisting of calcium carbonate lime sludge is obtained as a by-product. Calcium oxide is recovered from the lime sludge by combusting it in a long, rotating furnace. The chemical cycle requires considerable energy and is not particularly harmless to the environment. The chemical composition of a normal melt from a soda recovery unit is typically as follows:
______________________________________ Compound % of melt (approx.) ______________________________________ Sodium carbonate Na.sub.2 Co.sub.3 70 Sodium sulphide Na.sub.2 S 27 Sodium sulphate Na.sub.2 SO.sub.4 3 Sodium sulphite Na.sub.2 SO.sub.3 Sodium thiosulphate Na.sub.2 S.sub.2 O.sub.3 small quantities Sodium chloride NaCl ______________________________________
After causticizing, the chemical composition of a normal white liquor is typically as follows:
______________________________________ Compound Content in g/l (as NaOH) ______________________________________ NaOH 100 Na.sub.2 S 40 Na.sub.2 CO.sub.3 20 Na.sub.2 SO.sub.4 2.8 ______________________________________
The white liquor recovered through a conventional soda recovery unit process has a sulphidity of 25-30%. ##EQU1##
Swedish patent SE-C-8502731-6 describes an alternative process to the soda recovery unit technique, which is based on a gasification technique in which the organic substances in the black liquor are gasified in a first process in a pressurized reactor by means of "flash-pyrolysis" to provide CO, CO2, H2 and H2S, the residue obtained consisting primarily of the inorganic components of the spent liquor in solid or molten form having a composition corresponding to the melt from the soda recovery unit, i.e. mainly consisting of sodium carbonate and a small proportion of sodium sulphide. As with the soda recovery unit technique, this melt is dissolved and a green liquor is obtained which is treated in the same known manner as when the soda recovery unit technique is used.
The chemical composition of spent liquor from the sulphate pulp industry is typically as follows:
______________________________________ Sodium + Potassium 8.7 mol/kg DS black liquor Sulphur 1.8 mol/kg DS black liquor Chlorine 0.05 mol/kg DS black liquor Carbon 29.6 mol/kg DS black liquor Hydrogen 39.2 mol/kg DS black liquor Oxygen 21.0 mol/kg DS black liquor ______________________________________
The mole ratio Na:S is 4.88:1.
Under certain operating conditions, such as at a pressure of 1.5 bar abs. and a temperature of 950.degree. C. in the gasification reactor, a melt is obtained consisting of 59 per cent by weight of Na.sub.2 CO.sub.3 and 31 per cent by weight of Na.sub.2 S.
Through Swedish patent SE-C-465 039 it is known to continuously supply sulphur compounds to the black liquor in order to alter the composition of the melt. By altering the mole ratio Na:S to 1.5:1 by the addition of 4 mol sulphur/kg DS thick liquor a melt having high sulphidity consisting substantially of 14 per cent by weight of Na.sub.2 CO.sub.3 and 80 per cent by weight of Na.sub.2 S is obtained under otherwise identical operating conditions.
Due to its large content of sodium sulphide the melt thus consists for the most part of active digesting chemicals, i.e. white liquor.
However, the difficulty is to maintain the low mole ratio Na:S, and if possible to reduce it even further, in order to obtain a white liquor of nearly 100% sulphidity. The amount of available sulphur compounds in a sulphate factory, for instance, is limited and only a small portion of the total amount of black liquor can therefore be treated in a gasification process of this type, unless sulphur is supplied externally. In practice this is impossible to maintain continuously for sulphur balance reasons. It is therefore difficult to maintain a low mole ratio under certain operating conditions--as is accentuated by the following operating example.
Increased pressure displaces the reaction towards the formation of carbonate in the following equilibrium reaction: EQU Na.sub.2 CO.sub.3 +H.sub.2 S.revreaction.Na.sub.2 S+CO.sub.2 +H.sub.2 O.
When the gas produced is supplied to a gas turbine a higher pressure is desirable in the gasification reactor.
In this operating example, therefore, the melt obtained contains an even larger proportion of sodium carbonate and an even larger addition of sulphur compounds is necessary. If, for instance, the previously specified chemical composition of the black liquor and the amount of sulphur added remain the same but the pressure is increased from 1.5 bar abs. to 25 bar abs., i.e. the mole ratio Na:S of 1.5:1 is maintained, a melt is obtained consisting substantially of 52 per cent by weight of Na.sub.2 CO.sub.3 and 39 per cent by weight of Na.sub.2 S, i.e. a melt in which the carbonate content has increased from 14% to 52% by weight. This means that causticizing is still required and also that the sulphidity in the melt decreases, which can be a disadvantage.
It is clear from the above that if increased pressures in the gasification reactor are used, mole ratios are required which are lower the higher the pressure is, and increasing amounts of sulphur compounds must be added in order to displace the above reaction to the right. It will be recognized that the quantities of sulphur required would be unmanageably large, in particular in the upper pressure levels.